Starting mixture for a dielectric composition

ABSTRACT

The invention relates to the formulation of a starting mixture for an insulating composition, comprising a vitreous phase and several ceramic phases in the respective ratios by volume for the vitreous phase between 85 and 60% and for the ceramic phases between 15 and 40%, the vitreous phase being a silicate of zinc and alkaline earth. 
     According to an embodiment of the invention the starting mixture is remarkable in that the vitreous phase is constituted by the molar ratios of the following oxides: 
     30 to 55% of silicon (SiO 2 ), 15 to 30% of zinc oxide (ZnO), 0 to 20% of boric anhydride (B 2  O 3 ), 0 to 10% of alumina (Al 2  O 3 ), 15 to 40% of barium oxide (BaO), and in that the ceramic phases are constituted by the ratios by volume of the following oxides: 0 to 10% of cobalt oxide (Co 3  O 4 ), 5 to 20% of zinc oxide (ZnO), and 10 to 25% of lead oxide (PbO 2 ). 
     Application: Manufacture of microcircuits on colaminated metallic substrates.

BACKGROUND OF THE INVENTION

The invention relates to the formulation of a starting mixture for adielectric composition, comprising a vitreous phase and several ceramicphases in the respective ratios by volume for the vitreous phase between85 and 60% and for the ceramic phases between 15 and 40%, the vitreousphase being a silicate of zinc and alkaline earth.

The invention is is useful in the manufacture of power microcircuits ora support of a large surface area for components in the field ofintegrated circuits.

It is known from the article by J. P. DELLA MUSSIA in "Electroniqueactualites" of Mar. 9, 1984, p. 15, entitled "Les telecommunicationsetrangeres homologuent les co-lamines d'Imphy" to substitute on theconventional ceramic substrate which constitutes the support of theassembly of the integrated circuits a new conductive laminate, coveredby an electrically insulating layer.

This laminate is constituted by a substrate of an alloy of the INVARtype (nickel-iron alloy containing 36% of nickel), each of the surfacesof which is provided with a copper plating thus forming a succession ofthree layers, copper, invar, copper. The laminate has a coefficient ofexpansion which is adjustable during its manufacture due to the factthat this coefficient depends on the relative thickness of the threelayers.

A laminate in which the triplet Cu/INVAR/Cu has the thicknesspercentages 16/68/16, has a number of advantages. Among these advantagesare:

a coefficient of expansion, parallel to the surface, very near that ofalumina and various semiconductors, between 0° and 300° C.,

a thermal conductivity, parallel to the surface, approximately seventimes greater than that of alumina. (While the thermal conductivityperpendicularly to the surface is of the same order of magnitude as thatof alumina). An essential function of this laminate thus is to serve asa radiator for the circuits which are disposed thereon.

the possibility of providing substrates of large dimensions, which isnot the case with alumina;

the possibility of providing substrates as thin as the substrates ofalumina, for example 35 μm,

the possibility of soldering on the rear side a supplementary layer ofcopper in order to improve the radiation properties,

the resistance to shocks and

the possibility of mechanical or chemical piercing, folding, stamping orcutting.

Another laminate of the triplet Cu/Invar/Cu 16/68/16, in which the innerlayer has a corrugated form, possesses enhanced refrigerating properiesdue to internal circulation. It presents, due to this fact, an extraadvantage, and may serve as a substrate for circuits intended, forexample, for military applications, having to satisfy the standards 55°C.-+125°C.

However these laminates have the following disadvantages:

They are not electrically insulating at the surface.

Oxides are formed at the surfaces if they are not protected.

From these advantages and disadvantages it follows that the laminateshave been provided originally to serve as a support and radiator forceramic substrate the so called "chip carrier" of integrated circuits orfor power components or components of large dimensions, due to thecompatibility of the coefficients of expansion of the laminate and ofthese elements forming at the surface of the laminate an electricallyinsulating layer on which the circuit of these elements is formed.

It is known, as described in the above-mentioned article to manufacturethe insulating layer and the circuit by means of polymerisable organicmaterials.

However, this process has several disadvantages among which are:

the poor temperature behaviour of such materials, the polymerizationtemperature being of the order of 250° C.;

the poor mechanical rigidity of the polymerisable materials, which doesnot permit taking advantage of the great rigidity of the substrate;

the bad electrical conductivity of the conductive layers which, beingpolymers charged with silver, show the following:

    R≃30mΩ/□;

the fact that said conductors cannot be soldered;

the lack of resolution of the realized conductors, which does not permitof devising circuits of high integration density;

On the contrary a conductive silk-screening ink such as that describedin GB-PS 1,489,031 presents several advantages which include:

the possibility of withstanding high temperatures;

a high mechanical rigidity;

the resolution of conductors permitting of obtaining circuits with highintegration density and

the very good electrical conductivity of said layer of which theresistance is:

    R≃2mΩ/□.

However such a conductive silk-screening ink cannot be used directly onthe laminated substrate. An insulating layer which is also perfectlyadapted to the laminate and to the conductive ink is preferably providedbetween said latter and the first conductive layer.

Thus following the technical problem is present: In order to profit bythe advantages presented by the new laminated substrate while formingthereon circuits by means of a conductive silk-screening ink as has beendescribed it is desirable to provide an insulating silk-screening ink.(ε as low as possible) and which has:

A high firing temperature thereby being able to withstand the refiringof the envisaged conductive layer(s);

a coefficient of expansion near that of the laminate not only in therange of temperatures between 0° and 300° but also for temperaturesbetween 300° and the firing temperature of the ink;

a good adherence to the laminate and

finally the possibility of being fired under a non-oxidizing atmosphereso as to avoid oxidation of the laminate.

A starting mixture for an insulating composition for use in aninsulating silk-screening ink of high firing temperature under nitrogenis known from U.S. Pat. No. 4,323,652. According to this patent thismixture comprises a vitreous phase constituted by the molar ratios ofthe following oxides: from 30 to 55% of silicon (SiO₂), from 20 to 40%of zinc oxide (ZnO), from 0 to 20% of boric anhydride (B₂ O₃), from 0 to10% of alumina, from 5 to 40% of calcium anhydride (CaO), strontiumoxide (SrO) and barium oxide (BaO), as well as optionally from 0 to 10%of cobalt oxide (CoO) as a dye. Said mixture comprises also severalceramic phases such as zinc oxide and cobalt oxide.

According to this patent by use of such a mixture in the ratios byvolume between 85 and 60% for the vitreous phase and between 15 and 40%for the ceramic phases dispersed in an organic carrier, an insulatingsilk-screening ink may be obtained which is capable of being fired in ahigh temperature in nitrogen and compatible with the conductive ink asdescribed. However, this ink is suitable only for the manufacture ofhybrid microcircuits on ceramic substrates and always leads to adeformation of the substrate when it is used on the laminate.

In effect, in order to adapt said ink to the laminate substrate, allpossible tests of variation of the ratios of the constituents of the inksuggested by the above-cited patent have been made. None of these ratioshave proven to be successful.

For, of all the difficulties encountered by developing such an ink, themost complicated one to solve is the adaptation of the temperaturecoefficient of the ink to that of the laminate. At any rate the solutionto the technical problem described is not found in the use of an inkmnufactured according to the above-mentioned patent.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a newstarting mixture for an electrically insulating composition such asdefined in the opening paragraph which is characterized in that theceramic phases include zinc oxide (ZnO), cobalt oxide (Co₃ O₄) and leaddioxide (PbO₂).

By use of these ceramic phases the ink manufactured by dispersing such acomposition in an organic carrier, allows the technological problem tobe solved and in particular presents a coefficient of expansion which iswell adapted to the laminate substrate chosen.

More particularly according to the invention the starting mixture ischaracterized in that the vitreous phase is constituted by the followingmolar ratios 30 to 55% of silicon dioxide (SiO₂), 15 to 30% of zincoxide (ZnO), 0 to 20% of boric oxide anhydride (B₂ O₃), 0 to 10% ofalumina (Al₂ O₃), 15 to 40% of barium oxide (BaO), and in that theceramic phases are constituted by the volume ratios: 0 to 10% of cobaltoxide (Co₃ O₄), 5 to 20% of zinc oxide (ZnO), and 10 to 25% of leaddioxide (PbO₂).

BRIEF DESCRIPTION OF THE DRAWING

In the drawing FIG. 1 is a graph showing the relative elongation as afunction of temperature of a number of materials including an insulatingcomposition of the invention;

FIG. 2, FIG. 3 and FIG. 4 are each cross-sectional views of integratedcircuit assemblies employing a laminated substrate and covered with aninsulating layer formed of the dielectric composition of the invention.

DETAILED DESCRIPTION OF THE INVENTION

According to a preferred embodiment the starting mixture ischaracterized in that the molar ratios of the mixture is characterizedin that the molar ratios of the vitreous phase are chosen to be equal to45% of silicon dioxide (SiO₂), 20% of zinc oxide (ZnO), 5% of boricanhydride (B₂ O₃) 5% of alumina (Al₂ O₃), 25% of barium oxide (BaO).

Also according to a preferred embodiment the starting mixture ischaracterized in that it comprises in ratios by volume 68% theconstituent forming the vitreous phase, associated with a ceramic phaseof zinc oxide (ZnO) in the ratio by volume of 11%, a ceramic phase ofcobalt oxide (Co₃ O₄) in the ratio by volume of 2% and a ceramic phaseof lead dioxide (PbO₂) in the ratios by volume of 19%.

The silk-screening ink obtained starting from said composition has ahigh softening temperature which permits the ultimate deposition of hightemperature conductive inks. The coefficient of expansion of saidinsulating silk-screening ink is extremely close to the coefficient ofexpansion of the described colaminate 16/68/16, even up to temperaturesnear the solidification temperature of the ink. On the other hand thisink permits a firing under nitrogen which avoids the oxidation of thelaminate. Finally the layers manufactured by means of said ink present aperfect coherence with the surface of the laminate, a large flatness anda large delicacy permitting the ultimate deposition of conductive layersof high definition.

The invention will now be described in greater detail, by way ofexample, with reference to the accompanying drawings, in which

FIG. 1 is a graph showing relative elongations, as a function oftemperature, of the laminate (1), of alumina (2) of silicon dioxide (3),of gallium arsenide (4), and of the insulator of the invention (5).

FIG. 2 shows an assembly of the "chip carrier" type on a laminatedsubstrate.

FIG. 3 shows an assembly of a circuit manufactured on semiconductor orsemi-insulator on a laminated substrate.

FIG. 4 shows an assembly of discrete elements on a laminated substrate.

As shown in FIG. 1 the relative elongation of alumina (curve 2) is veryclose to the relative elongation of the laminate of the type 16/68/16between 0° and 300° C. (curve 1). Above this temperature range therelative elongation of the laminate in the directions parallel to itssurface increases very rapidly. Hence an insulating silk-screening inksuch as the ink described in U.S. Pat. No. 4,323,652 mentioned above therelative elongation of which follows that of alumina, would produce, onthe deposition face of the laminate during cooling after firing, ashrinkage which would produce the appearance of a convex rear surface.On the contrary, the silk-screening ink produced according to theinvention presents an average relative elongation near that of thecolaminate up to the firing temperature at approximately 530° C. (curve5 of FIG. 1). Hence, during cooling after firing no deformation at allis produced of the colaminated substrate.

The originality of the present invention hence resides in the fact ofusing as a ceramic phase a mixture of zinc oxide, cobalt oxide and leadoxide, the composition of the vitreous phase being hence slightlydifferent from the vitreous phases known in the art.

Examples of the glass (vitreous compositions) used are recited in thefollowing tables, the compositions A and B corresponding to twoembodiments (Table I) and table II giving the ranges of the possiblecompositions.

                  TABLE I                                                         ______________________________________                                        Vitreous phase: molar composition in %                                        Example A        Example B (preferred)                                        ______________________________________                                        ZnO          20      20                                                       SiO.sub.2    35      45                                                       B.sub.2 O.sub.3                                                                            20       5                                                       Al.sub.2 O.sub.3                                                                           10       5                                                       BaO          15      25                                                       ______________________________________                                        Starting mixture: composition in volume                                       Above vitreous phase   68%                                                                           ZnO     11%                                            Ceramic phases         Co.sub.3 O.sub.4                                                                       2%                                                                   PbO.sub.2                                                                             19%                                            ______________________________________                                        Example A        Example B                                                    ______________________________________                                        10.sup.7 α                                                                           73      80                                                       ρ (kg · m.sup.-3)                                                             3394    3187                                                     AP (°C.)                                                                            615     550                                                      DSP (°C.)                                                                           650     580                                                      ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        Vitreous phase: molar composition in %                                        ______________________________________                                        ZnO                  15 to 30%                                                SiO.sub.2            30 to 55%                                                B.sub.2 O.sub.3       0 to 20%                                                Al.sub.2 O.sub.3      0 to 10%                                                BaO                  15 to 40%                                                ______________________________________                                        Starting mixture: composition in volume                                       ______________________________________                                        Above vitreous phase 85 to 60%                                                                     ZnO      5 to 20%                                        Ceramic phases       Co.sub.3 O.sub.4                                                                       0 to 10%                                                             PbO.sub.2                                                                             10 to 25%                                        ______________________________________                                    

In addition to the composition for two examples, Table I also recitesthe voluminal mass ρ in kilograms per m³, the coefficient of expansionα×10⁷ per °C. between 20° and 320° C., an annealing temperature in °C.(AP) and the dilatometric softening point (DSP) in °C.

For a good manufacture of a silk screening ink according to the presentinvention, first a glass of the vitreous phase is formed in the molarratios corresponding to the ranges recited in Table II or in thespecific ratios given as preferred ratios (example B, Table I).

The glass thus obtained is pulverised. During this operation there areincorporated the powders of zinc oxide (ZnO), of cobalt (Co₃ O₄) andlead (PbO₂) in order to obtain a homogeneous mixture in the preferentialvolume ratios of 68% of glass, 11% of zinc oxide (ZnO), 2% of cobaltoxide (Co₃ O₄) and 19% of lead dioxide (PbO₂).

This pulverisation may be effected in a liquid medium, for examplewater. The result of the pulverisation is then dried and dispersed in anorganic carrier.

In so far as organic carrier is suitable to make said starting mixturesuitable for silk-screening, a solution may be used containing apolymer, for example a solution of ethyl cellulose in a terpineol, or amixture based on terpineol. Before firing the organic carrier may form10 to 40% by weight of silk screening ink. The ratios of the organiccarrier with respect to the ink are chosen as a function of the desiredrheological behaviour.

The organic carrier must be removed during firing. In order to avoidoxidation of the copper surfaces of the laminate, the ink prepared anddeposited on the substrate is subjected to firing under nitrogen. Theorganic carrier during such a firing can be destroyed only due to theoxygen supplied by the oxides of the ceramic phases and in particularCo₃ O₄.

Hence, with the object of applying oxygen, it is essential for thesupply of organic carrier that the lead be introduced in the form ofdioxide (PbO₂) in the ceramic phases.

Lead oxide (PbO) acts to permit the adjustment of the coefficient ofexpansion of the ink according to the values chosen.

However, this lead oxide (PbO) could not be introduced directly in thevitreous phase for in this case the quantity of oxygen supplied by theceramic phase Co₃ O₄ would be insufficient. It would thus be necessaryto adjust the ratios of the ceramic phases, and the adjustment of thecoefficient of expansion to the desired values would not be obtained anymore.

On the contrary, by introducing the lead in the form of the dioxide(PbO₂) in the ceramic phases, the quantities of oxygen used aresufficient to destroy the organic carrier during firing and theresulting lead oxide (PbO) at that moment enters into the vitreous phasepermitting the adjustment of the desired coefficient of expansion.

Finally, the composition according to the invention has an additionaladvantage. The vitreous phase is spontaneously devitrifiable due to itsbehaviour with zinc oxide. Said devitrification increases the meltingpoint of the layer by the formation of a dispersed crystalline phasewhich is less fusible. Said property avoids a spreading out of theinsulating layer during firing of the conductive layer depositedafterwards on its surface. On the other hand the zinc oxide constitutingone of the ceramic phases will reinforce said effect for it becomes adevitrifying agent after dissolution in the vitreous phase.

Various examples of devices of using the ink manufactured according tothe invention are possible.

A first such example is shown in FIG. 2. As shown in this figure, thedevice comprises a laminated substrate 20 composed of two copper layers1 and 3 adhering respectively to each face of a plate of Invar 2. On theupper layer 3 there is deposited an insulating silk screening layer 10of the invention according to a pattern which provides apertures topermit the connection of the rear side 25 of the casing 25, 26 to thesubstrate 20. This may be, for example, an aperture 5 slightly below thesurface of a casing or chip carrier 25, 26, destined to be fixed to thesubstrate afterwards. On the insulating layer 10 are then deposited aseries of layers which are alternately conductive and insulating and areformed by means by use of of silk screening ink of high temperature,using a pattern providing to constitute the connection circuit of thechip carrier with the output blocks of the device which are implanted onthe insulating layer 10 at the periphery of the laminated substrate 20.The first layer formed on the insulating layer 10 may be a conductivelayer 11 of the type described in GB-Pat. No. 1,489,031. The followinglayer may be an insulating layer 12 which must then imperatively be ofthe type described in the present invention for reasons of adapting tothe conductive layer on the one hand and to the coefficient of expansionof the substrate of the other hand, which reasons have already beenexplained hereinbefore. In this manner, up to five conductive layers ofthe type 11 may be realized separated by insulating layers according tothe invention. FIG. 2 thus shows the insulating layers 10, 12 separatingthe conductive layers 11, 13. A soldering paste of tin-lead (Sn-Pb60-40) 4 which can be silk-screened may then be deposited in theaperture 5 provided to receive the chip carrier 25, 26. The chip carrier25, 26 will then be fixed in the location provided by refusion of saidlayer. The chip carrier itself contains a microcircuit 27 manufacturedon silicon or gallium arsenide the outputs of which are made by thewires 8 and 18 connected to the output blocks 9 and 19 of the chipcarrier. These blocks themselves are connected to the conductive layersof the type of the layer 11 or 13 by the soldered joints 7 and 17, Thechip carrier is always provided on a conductive layer.

FIG. 3 shows a second embodiment of device employing the ink of theinvention. In this example the microcircuit 27 is directly fixed on thecolaminated substrate 20 either by means of a fusible silk-screeningglass or by gluing or by means of a gold-silicon alloy (Au-Si) 4, afterthe manufacture on the colaminate of a hybrid circuit having peripheralconnections constituted by a series of alternately insulating andconductive layers of the type of the layers 10 and 11 describedhereinbefore.

FIG. 4 shows a third embodiment of a device employing the ink of theinvention. In this example the laminate 20 protected by the insulatinglayer 10 according to the invention receives active elements such asdiodes 21 and transistors 22 and passive elements such as the resistor23, capacitor 24 and inductors all the interconnections of which aremade by means of silk-screening ink of the type of the layers 10 and 11described hereinbefore. The discrete elements may be connected by meansof layers 4 of fusible silk-screening glass or an alloy of gold-silicon(Au-Si) as described above. The active elements may be manufactured onsemiconductor substrates, silicon (Si) for example, or semi-insulatingsubstrates, for example, gallium arsenide (GaAs). The passive elementsmay be manufactured on alumina.

As shown in FIG. 1, the curves of relative elongation of silicon (curve3), of alumina (curve 2), of gallium arsenide (curve 4) teach that thefixation of said materials is perfectly compatible with the laminatedsubstrate the relative elongation of which is shown by curve 1, this upto approximately 200° C. for silicon and approximately 300° C. forgallium arsenide and alumina.

By directly fixing the components, microcircuits or discrete elements onthe colaminate as shown in the second and third examples, the improvedconditions for the cooling of said components may be used to advantage.

In each of the three described examples the method of manufacturingcomprises the steps of:

1. Depositing the insulating layer according to the invention bysilk-screening through a screen according to a chosen pattern and firingsaid layer.

2. Depositing a conductive layer, for example such as described in GBPat. No. 1,489,031 through a silk-screening screen according to a chosenpattern and firing said layer.

3. Repetition of the steps 1 and 2 until the circuit described isobtained.

4. Fixing the elements chip-carrier, microcircuits or discrete elementsby means of suitable silk-screening soldering pastes.

5. Optionally locally protecting the insulated elements, other than thechip-carrier, by encapsulation. At this level such an encapsulation maybe made without any problems by a drop of polymer 3 since it relates toa simple protection and not a part of the circuit which must present acertain rigidity.

It is to be noted that circuits thus manufactured have shown for theinsulating layer according to the invention a dielectric constant ε_(r)=12 measured at 1 MHz, dielectric losses lower than 3×10⁻² and aresistance to the upper insulation at 10¹¹ Ω for a capacitor of 5×5 mm².

It will be obvious that numerous variations are possible within thecomposition ranges given or in the form of manufacturing the circuitsusing said insulating composition, or in the equivalent substituentswithout therefor departing from the scope of the present invention.

I claim:
 1. A starting mixture suitable for forming a dielectriccomposition consisting essentially of 85%-60% by volume of a vitreousphase and 15%-40% by volume of ceramic phases, said vitreous phasehaving the following composition by mol.%:30-55% SiO₂, 15-30% ZnO,15-40% BaO, 0-20% B₂ O₃ and 0-10% Al₂ O₃ and said ceramic phasesconsisting essentially in volume percent of: ZnO-5-20%, Co₃ O₄ -0-10%and PbO₂ -10-25%.
 2. A starting mixture as claimed in claim 1,characterized in that the molar ratios of the vitreous phase are chosento be equal to 45% of silicon (SiO₂), 20% of zinc oxide (ZnO), 5% ofboric anhydride (B₂ O₃), 5% of alumina (Al₂ O₃) and 25% of oxide (BaO).3. A starting mixture as claimed in claim 1, characterized in that itconsists essentially of 68% by volume of the vitreous phase, a ceramicphase of zinc oxide (ZnO) in 11% by volume, a ceramic phase of cobaltoxide (Co₃ O₄) in 2% by volume and a ceramic phase of lead dioxide(PbO₂) in 19% by volume.
 4. A starting mixture as claimed in claim 2,characterized in that it consists essentially of the vitreous phase in68% by volume, a ceramic phase of zinc oxide (ZnO) in 11% by volume, aceramic phase of cobalt oxide (Co₃ O₄) in 2% by volume and a ceramicphase of lead dioxide (PbO₂) in 19% by volume.